Field of the Invention
The present invention relates to an object information acquiring apparatus and a control method for the same.
Description of the Related Art
In an ultrasound wave diagnostic apparatus that achieves medical ultrasound wave imaging, it is possible to generally represent the spatial resolution in the depth direction, in the case where an image is formed by pulse echo method, by (nλ)/2, where λ is the wavelength of an ultrasound wave and n is the number of transmitted waves. For example, the spatial resolution in the case where two wavelengths of ultrasound waves with a center frequency of 12 MHz have been transmitted is approximately 0.13 mm.
The pulse echo method will be described. First, when an ultrasound wave pulse (elastic wave) is transmitted to an object by a probe or the like, an ultrasound wave is reflected back in accordance with the acoustic impedance within the object. Next, this reflected wave is received, and an envelope of the waveform of the reflected wave is acquired. By the envelope being converted into and displayed in a brightness value, brightness information on a scanning line in a direction in which the ultrasound wave has been transmitted and received is obtained. By repeating transmission and reception of an ultrasound wave with respect to a plurality of directions or positions within the object, brightness information on a plurality of scanning lines can be acquired. By arranging the brightness information on the a plurality of scanning lines, imaging of the inside of the object is possible. Generally, in an ultrasound wave diagnostic apparatus, a plurality of ultrasound wave/electricity converting elements are used, and a temporal difference between the waveforms of the respective elements is added for focusing within an object in both transmission and reception.
Although the spatial resolution of approximately 0.13 mm in the depth direction can be achieved by using the pulse echo method as described above, there are demands for higher spatial resolution. For example, if the layer structure in a blood vessel wall of a carotid artery can be observed in further detail, there would supposedly be a contribution to early detections of arteriosclerosis or the like.
A technique of improving the spatial resolution in such depth direction, i.e., direction in which an ultrasound wave is transmitted and received, is described in Japanese Patent Application Laid-open No. 2010-183979 and “Hirofumi Taki, Kousuke Taki, Takuya Sakamoto, Makoto Yamakawa, Tsuyoshi Shiina and Toru Sato: Conf Proc IEEE Eng Med Biol Soc. 2010; 1:5298-5301.” In these documents, frequency domain interferometry (FDI) and Capon method that is adaptive signal processing are applied, and the result of visualizing a blood vessel is shown.
As described above, a general ultrasound wave diagnostic apparatus forms an image by acquiring an envelope of the received waveform for each scanning line. In the case of further improving the spatial resolution in the depth direction by applying FDI and the Capon method to the received waveform, the existence of a plurality of reflection layers is expected in a range of signals in the depth direction (within processing range) that is cut out in order to perform processing of FDI. In the case where a plurality of close reflection layers exist, there is a high possibility that reflected waves from the reflection layers have a high correlation with each other in the pulse echo method, since reflected waves with respect to one transmission waveform are received. It is known that, when adaptive signal processing of the Capon method or the like is directly applied with respect to a plurality of reflection waveforms having a high correlation in this manner, an unexpected behavior such as cancelling out a desired signal is performed. In order to reduce and prevent such influence by a signal having correlation (correlative interference wave), there is an approach of using the frequency averaging technique in combination. Accordingly, it is possible to apply FDI and the Capon method with respect to a signal obtained with the general pulse echo method.    Patent Literature 1: Japanese Patent Application Laid-open No. 2010-183979    Non Patent Literature 1: Hirofumi Taki, Kousuke Taki, Takuya Sakamoto, Makoto Yamakawa, Tsuyoshi Shiina and Toru Sato: Conf Proc IEEE Eng Med Biol Soc. 2010; 1:5298-5301